This invention relates generally to circuit interrupters and more particularly to circuit interrupters in an automotive environment in which the electrical system operating voltage has been elevated above the conventional 14 Vdc, for example to 42 Vdc, in order to meet future power demands.
Circuit interrupter devices for electrical motors are commonly connected in motor winding circuits to respond to overload currents which result when certain fault conditions occur, thereby to interrupt the high currents which would tend to cause rapid overheating of the motor windings. It is also desirable to arrange such devices to be directly responsive to increases in winding temperature to interrupt the winding circuits to protect the windings against relatively slower build-up of winding temperatures.
Circuit interrupt devices conventionally used for automotive applications typically comprise a generally flat open-ended electrically conductive metal can which has a flange around its open end. A generally flat, electrically conductive metal lid is attached to and electrically isolated from the open end of the can by an electrically insulating gasket which fits between the lid and the can flange. A thermally responsive bimetallic member has one end welded or otherwise secured to the inside surface of the bottom of the can. The bimetallic member extends in cantilever relation from the can bottom and carries a movable contact at its distal end, the member being adapted for snap-acting movement between two opposite configurations in response to temperature change to engage and disengage the movable contact with a stationary contact mounted on the inner surface of the lid thereby to open and close an electrical circuit between device terminals extending from the can and lid. It is also common that such motor protectors incorporate a heating element of selected electrical resistance material to be energized by current passing through the protector for heating the bimetal, along with heat generated in the bimetallic member itself. One such arrangement includes a lid member which is separated into two parts spaced from one another and with an end of the heater element connected to each part. In addition to providing desired current sensitivity the heating element aids in maintaining the bimetallic member above its reset temperature for an extended period of time so that the protector device is adapted to cycle on and off at a relatively slow rate while the fault condition causing the overload current persists and aids in providing a relatively long service life.
Circuit interrupters as described above presently used in automotive applications have been designed for operation at 16 Vdc during locked rotor conditions. The contact gaps provided in such protectors are insufficient to interrupt the electric arc generated at the proposed voltage and ampere levels which would cause the contacts to melt across the open gap as the bimetallic member attempts to interrupt the circuit. The welded contact interface would sustain locked rotor conditions and permit a hazardous high temperature condition to be created within the motor.
Conventional protective devices as described above have been designed to function at ampere levels between 45 and 4 amps at 14 Vdc for applications such as windshield wiper, window lift and seat adjusting motors. Increasing the electrical supply system to 42 V would reduce these values by a factor of three, producing lock rotor ampere ratings between 15 and 1.3 amps at 42 Vdc for these same applications. The electrical resistance needed to produce the I2r heating necessary to heat the device to its actuation temperature would increase generally by a factor of nine. Merely increasing the size of the bimetal member to produce the required contact gap is not suitable because this would reduce current sensitivity and result in an increased product envelope causing assembly problems due to the limited space available for protectors in motor housings, particularly in the automotive environment.
An object of the present invention is the provision of an improved circuit interrupter having sufficient contact gap to interrupt the arc in an elevated electrical supply system while achieving current sensitivity at a fraction of the ampere levels of conventional protective devices while at the same time maintaining a small envelope for the device. Another object of the invention is to provide an improved motor protector for use in protecting relatively small electrical motors which is reliable, low in cost and one which has a relatively long service life.
Briefly described, a circuit interrupter made in accordance with a first preferred embodiment of the invention comprises a bimetallic member having a motion amplifying extended length portion extending from a dished shaped portion for providing snap action of the member between two oppositely configured positions and a movable contact at a free end of the extended length portion adapted to move into and out of electrical engagement with a stationary contact. The extended length portion includes a stiffening feature to minimize the portion""s displacement associated with the bimetal member""s flexivity and normal temperature variation. The stiffening feature increases the mechanical spring rate of the bimetallic member thereby promoting larger snap acting displacement characteristics which otherwise would be lost to mechanical wind-up and results in an optimization of the slope of the calibration curve.
The stiffening feature may take one or more of several forms including folding the outer edge of the extension portion relative to a plane in which the remainder of the extension portion lies, for example, by 90xc2x0 or 180xc2x0 thereto, longitudinally extending rib or ribs formed in the extended length portion and forming the extended length portion with one or more cut-out portions which also increases current sensitivity of the bimetallic member. The extended length portion may have a generally uniform width extending from the dished portion to the movable contact or it may be tapered so that the width narrows in the direction going from the dished portion to the movable contact. In certain embodiments the width of the extended length portion at the dished portion is wider than the generally circular dished shaped portion. In a preferred embodiment, each side of a tapered width extended length portion is generally tangent to the generally circular dished shaped portion.
The bimetallic member of at least certain ones of the preferred embodiments is formed with another extended length portion for attachment of the bimetallic member in a housing and according to a preferred embodiment this portion is also provided with a stiffening feature comprising opposed folded edge portions of the bimetallic member to further minimize mechanical wind-up and thermal creep thereby increasing snap-open gap and increasing resonant frequency to avoid loss of continuity during vibration exposure.
In another preferred embodiment, the bimetallic element has an extended length portion including a stiffening feature which extends from the dished shaped portion in two opposite directions, each with a movable electrical contact which is adapted to move into and out of electrical engagement with a respective stationary electrical contact mounted in a housing. The dished shaped portion may be provided with a centrally disposed aperture which receives a mounting member therethrough for mounting the member in the housing.